Nanostructured free-electron metals gained a lot of attention due to their interesting optical properties that are based on resonant excitation of surface plasmons (SP). SPs are waves of oscillating surface charge density traveling along the metal surface. The manipulation of SPs properties by tailoring the geometric parameters of the nanostruture is a promising approach for the development of plasmonic-based applications such as device fabrication, imaging technologies, and information processing. Sub-wavelength nanohole arrays in thin gold films show an especially interesting optical phenomenon called “extraordinary optical transmission” (EOT). These nanostructures are more transparent at certain wavelengths than expected by the classical aperture theory and can be used for enhanced spectroscopy as well as chemical sensing. Sensors based on this technology could offer several advantages such as higher spatial resolution, greater reproducibility, and more convenient experimental geometries.
In recent years, colloidal nanolithography has made considerable progress (Xia et al., Adv. Mater. 2000, 693-713; Yang et al., Small 2007, 2, 458-475) and has also been used for producing nanostructures showing an EOT effect. In particular, Murray et al. (Physical Review B 69, 165407-1-165407-7 (2004) used an ordered monolayer of polystyrene nanospheres as a deposition mask through which silver was deposited by thermal evaporation. By reactive ion etching of the nanospheres in an oxygen plasma prior to silver deposition, arrays consisting of silver particles of increasing size were produced which—with increasing etching time—gradually merged into a continuous metal film perforated by a periodic nanohole array in a silver film. This method requires a rather expensive equipment and is laborious due to the plasma etching step, which has to be optimized for every plasma machine again. Moreover, the surface of the polystyrene spheres is increasingly frayed out during plasma exposure leading to a loss of the spherical shape of the polymer mask. The surface roughness of the polymer spheres increases proportionally to the process duration which has a direct effect on the quality of the pore rims of the metal layer deposited thereafter. In view of the drawbacks of this method as well as that of other nanolithography techniques of the art, periodic arrays of nanoholes in opaque metal films are usually fabricated by focused ion beam, electron beam lithography, and photolithography until now (De Leebeeck et al., Anal. Chem. 2007, 79, 4094-4100; Sharpe et al., Anal. Chem. 2008, 80, 2244-2249). These techniques are limited by either low resolution (photolithography) or low throughput (e-beam lithography, focused ion beam lithography). They are time-consuming, expensive, and provide only small nanostructured areas.
Thus, an object of the present invention is to provide improved methods for producing highly ordered arrays of nanoholes in metallic films on a substrate which are fast, cost-efficient and simple to perform without the need of expensive equipment, for example in any standard chemical laboratory. A further object is to provide large and highly ordered arrays of nanoholes in metallic films on a substrate, with the size and lattice constant of the nanoholes being easily adjustable over a broad range.
Said objects are achieved by providing a novel method involving colloidal nanolithography for producing highly ordered arrays of nanoholes in metallic films on a substrate according to the present invention and by providing the highly ordered array of nanoholes according to the present invention.